Thermoset nanocomposite particles, processing for their production, and their use in oil and natural gas drilling applications

ABSTRACT

Use of two different methods, either each by itself or in combination, to enhance the stiffness, strength, maximum possible use temperature, and environmental resistance of thermoset polymer particles is disclosed. One method is the application of post-polymerization process steps (and especially heat treatment) to advance the curing reaction and to thus obtain a more densely crosslinked polymer network. The other method is the incorporation of nanofillers, resulting in a heterogeneous “nanocomposite” morphology. Nanofiller incorporation and post-polymerization heat treatment can also be combined to obtain the benefits of both methods simultaneously. The present invention relates to the development of thermoset nanocomposite particles. Optional further improvement of the heat resistance and environmental resistance of said particles via post-polymerization heat treatment; processes for the manufacture of said particles; and use of said particles in the construction, drilling, completion and/or fracture stimulation of oil and natural gas wells are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/495,454, filed 24 Sep. 2014, now allowed, which is acontinuation of U.S. patent application Ser. No. 13/717,636, filed 17Dec. 2012, (now abandoned), which is a continuation of U.S. patentapplication Ser. No. 13/629,018, filed 27 Sep. 2012, (now issued as U.S.Pat. No. 8,466,093), which is a continuation of U.S. patent applicationSer. No. 13/353,542, filed 19 Jan. 2012, (now issued as U.S. Pat. No.8,278,373), which in turn is a continuation of prior U.S. applicationSer. No. 13/340,080, filed Dec. 29, 2011, (now issued as U.S. Pat. No.8,455,403), which in turn is a continuation of prior U.S. applicationSer. No. 12/980,510, filed Dec. 29, 2010, (now issued as U.S. Pat. No.8,088,718), which in turn is a division of prior U.S. application Ser.No. 12/870,076, filed 27 Aug. 2010 (now issued as U.S. Pat. No.7,902,125), which in turn is a divisional of prior U.S. application Ser.No. 11/323,031, filed 30 Dec. 2005, (now issued as U.S. Pat. No.7,803,740), which in turn claims priority from U.S. ProvisionalApplication Ser. No. 60/640,965, filed 30 Dec. 2004, all of which arehereby incorporated by reference herein in their entireties.

FIELD OF THE INVENTION

The present invention relates to lightweight thermoset polymernanocomposite particles, to processes for the manufacture of suchparticles, and to applications of such particles. The particles of theinvention contain one or optionally more than one type of nanofillerthat is intimately embedded in the polymer matrix. It is possible to usea wide range of thermoset polymers and nanofillers as the mainconstituents of the particles of the invention, and to produce saidparticles by means of a wide range of fabrication techniques. Withoutreducing the generality of the invention, in its currently preferredembodiments, the thermoset matrix consists of a terpolymer of styrene,ethyvinylbenzene and divinylbenzene; particulate carbon black ofnanoscale dimensions is used as the nanofiller, suspensionpolymerization is performed in the presence of the nanofiller, andoptionally post-polymerization heat treatment is performed with theparticles still in the reactor fluid that remains after the suspensionpolymerization to further advance the curing of the matrix polymer. Whenexecuted in the manner taught by this patent, many properties of boththe individual particles and packings of said particles can be improvedby the practice of the invention. The particles exhibit enhancedstiffness, strength, heat resistance, and resistance to aggressiveenvironments; as well as the improved retention of high conductivity ofliquids and gases through packings of said particles in aggressiveenvironments under high compressive loads at elevated temperatures. Thethermoset polymer nanocomposite particles of the invention can be usedin many applications. These applications include, but are not limitedto, the construction, drilling, completion and/or fracture stimulationof oil and natural gas wells; for example, as a proppant partialmonolayer, a proppant pack, an integral component of a gravel packcompletion, a ball bearing, a solid lubricant, a drilling mudconstituent, and/or a cement additive.

BACKGROUND

The background of the invention can be described most clearly, and hencethe invention can be taught most effectively, by subdividing thissection in three subsections. The first subsection will provide somegeneral background regarding the role of crosslinked (and especiallystiff and strong thermoset) particles in the field of the invention. Thesecond subsection will describe the prior art that has been taught inthe patent literature. The third subsection will provide additionalrelevant background information selected from the vast scientificliterature on polymer and composite materials science and chemistry, tofurther facilitate the teaching of the invention.

A. General Background

Crosslinked polymer (and especially stiff and strong thermoset)particles are used in many applications requiring high stiffness, highmechanical strength, high temperature resistance, and/or high resistanceto aggressive environments. Crosslinked polymer particles can beprepared by reacting monomers or oligomers possessing three or morereactive chemical functionalities, as well as by reacting mixtures ofmonomers and/or oligomers at least one ingredient of which possessesthree or more reactive chemical functionalities.

The intrinsic advantages of crosslinked polymer particles over polymerparticles lacking a network consisting of covalent chemical bonds insuch applications become especially obvious if an acceptable level ofperformance must be maintained for a prolonged period (such as manyyears, or in some applications even several decades) under the combinedeffects of mechanical deformation, heat, and/or severe environmentalinsults. For example, many high-performance thermoplastic polymers,which have excellent mechanical properties and which are hence usedsuccessfully under a variety of conditions, are unsuitable forapplications where they must maintain their good mechanical propertiesfor many years in the presence of heat and/or chemicals, because theyconsist of assemblies of individual polymer chains. Over time, thedeformation of such assemblies of individual polymer chains at anelevated temperature can cause unacceptable amounts of creep, andfurthermore solvents and/or aggressive chemicals present in theenvironment can gradually diffuse into them and degrade theirperformance severely (and in some cases even dissolve them). Bycontrast, the presence of a well-formed continuous network of covalentbonds restrains the molecules, thus helping retain an acceptable levelof performance under severe use conditions over a much longer timeperiod.

Oil and natural gas well construction activities, including drilling,completion and stimulation applications (such as proppants, gravel packcomponents, ball bearings, solid lubricants, drilling mud constituents,and/or cement additives), require the use of particulate materials, inmost instances preferably of as nearly spherical a shape as possible.These (preferably substantially spherical) particles must generally bemade from materials that have excellent mechanical properties. Themechanical properties of greatest interest in most such applications arestiffness (resistance to deformation) and strength under compressiveloads, combined with sufficient “toughness” to avoid the brittlefracture of the particles into small pieces commonly known as “fines”.In addition, the particles must have excellent heat resistance in orderto be able to withstand the combination of high compressive load andhigh temperature that normally becomes increasingly more severe as onedrills deeper. In other words, particles that are intended for usedeeper in a well must be able to withstand not only the higheroverburden load resulting from the greater depth, but also the highertemperature that accompanies that higher overburden load as a result ofthe nature of geothermal gradients. Finally, these materials must beable to withstand the effects of the severe environmental insults(resulting from the presence of a variety of hydrocarbon and possiblysolvent molecules as well as water, at simultaneously elevatedtemperatures and compressive loads) that the particles will encounterdeep in an oil or natural gas well. The need for relatively lightweighthigh performance materials for use in these particulate components inapplications related to the construction, drilling, completion and/orfracture stimulation of oil and natural gas wells thus becomes obvious.Consequently, while such uses constitute only a small fraction of theapplications of stiff and strong materials, they provide fertileterritory for the development of new or improved materials andmanufacturing processes for the fabrication of such materials.

We will focus much of the remaining discussion of the background of theinvention on the use of particulate materials as proppants. One keymeasure of end use performance of proppants is the retention of highconductivity of liquids and gases through packings of the particles inaggressive environments under high compressive loads at elevatedtemperatures.

The use of stiff and strong solid proppants has a long history in theoil and natural gas industry. Throughout most of this history, particlesmade from polymeric materials (including crosslinked polymers) have beenconsidered to be unsuitable for use by themselves as proppants. Thereason for this prejudice is the perception that polymers are toodeformable, as well as lacking in the ability to withstand thecombination of elevated compressive loads, temperatures and aggressiveenvironments that are commonly encountered in oil and natural gas wells.Consequently, work on proppant material development has focused mainlyon sands, on ceramics, and on sands and ceramics coated by crosslinkedpolymers to improve some aspects of their performance. This situationhas prevailed despite the fact that most polymers have densities thatare much closer to that of water so that in particulate form they can betransported much more readily into a fracture by low-density fracturingor carrier fluids such as unviscosified water.

Nonetheless, the obvious practical advantages [see a review by Edgeman(2004)] of developing the ability to use lightweight particles thatpossess almost neutral buoyancy relative to water have stimulated aconsiderable amount of work over the years. However, as will be seenfrom the review of the prior art provided below, progress in this fieldof invention has been very slow as a result of the many technicalchallenges that exist to the successful development of cost-effectivelightweight particles that possess sufficient stiffness, strength andheat resistance.

B. Prior Art

The prior art can be described most clearly, and hence the invention canbe placed in the proper context most effectively, by subdividing thissection into four subsections. The first subsection will describe priorart related to the development of “as-polymerized” thermoset polymerparticles. The second subsection will describe prior art related to thedevelopment of thermoset polymer particles that are subjected topost-polymerization heat treatment. The third subsection will describeprior art related to the development of thermoset polymer compositeparticles where the particles are reinforced by conventional fillers.The fourth subsection will describe prior art related to the developmentof ceramic nanocomposite particles where a ceramic matrix is reinforcedby nanofillers.

1. “As-Polymerized” Thermoset Polymer Particles

As discussed above, particles made from polymeric materials havehistorically been considered to be unsuitable for use by themselves asproppants. Consequently, their past uses in proppant materials havefocused mainly on their placement as coatings on sands and ceramics, inorder to improve some aspects of the performance of the sand and ceramicproppants.

Significant progress was made in the use of crosslinked polymericparticles themselves as constituents of proppant formulations in priorart taught by Rickards, et al. (U.S. Pat. No. 6,059,034; U.S. Pat. No.6,330,916). However, these inventors still did not consider or describethe polymeric particles as proppants. Their invention only related tothe use of the polymer particles in blends with particles of moreconventional proppants such as sands or ceramics. They taught that thesand or ceramic particles are the proppant particles, and that the“deformable particulate material” consisting of polymer particles mainlyserves to improve the fracture conductivity, reduce the generation offines and/or reduce proppant flowback relative to the unblended sand orceramic proppants. Thus while their invention differs significantly fromthe prior art in the sense that the polymer is used in particulate formrather than being used as a coating, it shares with the prior art thelimitation that the polymer still serves merely as a modifier improvingthe performance of a sand or ceramic proppant rather than beingconsidered for use as a proppant in its own right.

Bienvenu (U.S. Pat. No. 5,531,274) disclosed progress towards thedevelopment of lightweight proppants consisting of high-strengthcrosslinked polymeric particles for use in hydraulic fracturingapplications. However, embodiments of this prior art, based on the useof styrene-divinylbenzene (S-DVB) copolymer beads manufactured by usingconventional fabrication technology and purchased from a commercialsupplier, failed to provide an acceptable balance of performance andprice. They cost far more than the test standard (Jordan sand) whilebeing outperformed by Jordan sand in terms of the liquid conductivityand liquid permeability characteristics of their packings measuredaccording to the industry-standard API RP 61 testing procedure. [Thisprocedure is described by the American Petroleum Institute in itspublication titled “Recommended Practices for Evaluating Short TermProppant Pack Conductivity” (first edition, Oct. 1, 1989).] The need touse a very large amount of an expensive crosslinker (50 to 80% by weightof DVB) in order to obtain reasonable performance (not too inferior tothat of Jordan Sand) was a key factor in the higher cost thataccompanied the lower performance.

The most advanced prior art in stiff and strong crosslinked polymerparticle technologies for use in applications in oil and natural gasdrilling was developed by Albright (U.S. Pat. No. 6,248,838) who taughtthe concept of a “rigid chain entanglement crosslinked polymer”. Insummary, the reactive formulation and the processing conditions weremodified to achieve “rapid rate polymerization”. While not improving theextent of covalent crosslinking relative to conventional isothermalpolymerization, rapid rate polymerization results in the “trapping” ofan unusually large number of physical entanglements in the polymer.These additional entanglements can result in a major improvement of manyproperties. For example, the liquid conductivities of packings of S-DVBcopolymer beads with w_(DVB)=0.2 synthesized via rapid ratepolymerization are comparable to those that were found by Bienvenu (U.S.Pat. No. 5,531,274) for packings of conventionally produced S-DVB beadsat the much higher DVB level of w_(DVB)=0.5. Albright (U.S. Pat. No.6,248,838) thus provided the key technical breakthrough that enabled thedevelopment of the first generation of crosslinked polymer beadspossessing sufficiently attractive combinations of performance and pricecharacteristics to result in their commercial use in their own right assolid polymeric proppants.

2. Heat-Treated Thermoset Polymer Particles

There is no prior art that relates to the development of heat-treatedthermoset polymer particles for use in oil and natural gas wellconstruction applications. One needs to look into another field oftechnology to find prior art of some relevance. Nishimori, et. al.(JP1992-22230) focused on the development of particles for use in liquidcrystal display panels. They taught the use of post-polymerization heattreatment to increase the compressive elastic modulus of S-DVB particlesat room temperature. They only claimed compositions polymerized fromreactive monomer mixtures containing 20% or more by weight of DVB orother crosslinkable monomer(s) prior to the heat treatment. They statedexplicitly that improvements obtained with lower weight fractions of thecrosslinkable monomer(s) were insufficient and that hence suchcompositions were excluded from the scope of their patent.

3. Thermoset Polymer Composite Particles

This subsection will be easier to understand if it is further subdividedinto two subsections. As was discussed above, the prior art on the useof polymers as components of proppant particles has focused mainly onthe development of thermoset polymer coatings for rigid inorganicmaterials such as sand or ceramic particles. These types ofheterogeneous (composite) particles will be discussed in the firstsubsection. Composite particles where the thermoset polymer plays a rolethat goes beyond that of a coating will be discussed in the secondsubsection.

a. Thermoset Polymers as Coatings

The prior art discussed in this subsection is mainly of interest forhistorical reasons, as examples of the evolution of the use of thermosetpolymers as components in composite proppant particles.

Underdown, et al. (U.S. Pat. No. 4,443,347) and of Glaze, et al. (U.S.Pat. No. 4,664,819) taught the coating of particles such as silica sandor glass beads with a thermoset polymer (such as a phenol-formaldehyderesin) that is cured fully (in their terminology, “pre-cured”) prior tothe injection of a proppant charge consisting of such particles into awell.

An interesting alternative coating technology was taught by Graham, etal. (U.S. Pat. No. 4,585,064) who developed resin-coated particlescomprising a particulate substrate, a substantially cured inner resincoating, and a heat-curable outer resin coating. According to theirteaching, the outer resin coating should cure, and should thus enablethe particles to form a coherent mass possessing the desired level ofliquid conductivity, under the temperatures and compressive loads foundin subterranean formations. However, it is not difficult to anticipatethe many technical difficulties that can arise in attempting to reducesuch an approach reliably and consistently to practice.

b. Thermoset Polymers as Matrix Phase Containing Dispersed FinelyDivided Filler Material

McDaniel, et al. (U.S. Pat. No. 6,632,527) describes composite particlesmade of a binder and filler; for use in subterranean formations (forexample, as proppants and as gravel pack components), in waterfiltration, and in artificial turf for sports fields. The fillerconsists of finely divided mineral particles that can be of anyavailable composition. Fibers are also used in some embodiments asoptional fillers. The sizes of the filler particles are required to fallwithin the range of 0.5 microns to 60 microns. The proportion of fillerin the composite particle is very large (60% to 90% by volume). Thebinder formulation is required to include at least one member of thegroup consisting of inorganic binder, epoxy resin, novolac resin, resoleresin, polyurethane resin, alkaline phenolic resole curable with ester,melamine resin, urea-aldehyde resin, urea-phenol-aldehyde resin, furans,synthetic rubber, and/or polyester resin. The final thermoset polymercomposite particles of the required size and shape are obtained by asuccession of process steps such as the mixing of a binder stream with afiller particle stream, agglomerative granulation, and the curing ofgranulated material streams.

4. Ceramic Nanocomposite Particles

Nguyen, et al. (U.S. 20050016726) taught the development of ceramicnanocomposite particles comprising a base material (present at roughly50% to 90% by weight) and at least one nanoparticle material (present atroughly 0.1% to 30% by weight). Optionally, a polymeric binder, anorganosilane coupling agent, and/or hollow microspheres, can also beincluded. The base material comprises clay, bauxite, alumina, silica, ormixtures thereof. It is stated that a suitable method for forming thecomposite particulates from the dry ingredients is to sinter by heatingat a temperature of between roughly 1000° C. and 2000° C., which is aceramic fabrication process. Given the types of formulation ingredientsused as base materials by Nguyen, et al. (U.S. 20050016726), andfurthermore the fact that even if they were to incorporate a polymericbinder in an embodiment of their invention said polymeric binder wouldnot retain its normal chemical composition and polymer chain structurewhen a particulate is sintered by heating it at a temperature of between1000° C. and about 2000° C., their composite particulates consist of thenanofiller(s) dispersed in a ceramic matrix.

C. Scientific Literature

The development of thermoset polymer nanocomposites requires theconsideration of a vast and multidisciplinary range of polymer andcomposite materials science and chemistry challenges. It is essential toconvey these challenges in the context of the fundamental scientificliterature.

Bicerano (2002) provides a broad overview of polymer and compositematerials science that can be used as a general reference for mostaspects of the following discussion. Many additional references willalso be provided below, to other publications which treat specificissues in greater detail than what could be accommodated in Bicerano(2002).

1. Selected Fundamental Aspects of the Curing of Crosslinked Polymers

It is essential, first, to review some fundamental aspects of the curingof crosslinked polymers, which are applicable to such polymersregardless of their form (particulate, coating, or bulk).

The properties of crosslinked polymers prepared by standardmanufacturing processes are often limited by the fact that suchprocesses typically result in incomplete curing. For example, in anisothermal polymerization process, as the glass transition temperature(T_(g)) of the growing polymer network increases, it may reach thepolymerization temperature while the reaction is still in progress. Ifthis happens, then the molecular motions slow down significantly so thatfurther curing also slows down significantly. Incomplete curing yields apolymer network that is less densely crosslinked than the theoreticallimit expected from the functionalities and relative amounts of thestarting reactants. For example, a mixture of monomers might contain 80%DVB by weight as a crosslinker but the final extent of crosslinking thatis attained may not be much greater than what was attained with a muchsmaller percentage of DVB. This situation results in lower stiffness,lower strength, lower heat resistance, and lower environmentalresistance than the thermoset is capable of manifesting when it is fullycured and thus maximally crosslinked.

When the results of the first scan and the second scan of S-DVB beadscontaining various weight fractions of DVB (w_(DVB)), obtained byDifferential Scanning calorimetry (DSC), as reported by Bicerano, et al.(1996) (see FIG. 1) are compared, it becomes clear that the lowperformance and high cost of the “as purchased” S-DVB beads utilized byBienvenu (U.S. Pat. No. 5,531,274) are related to incomplete curing.This incomplete curing results in the ineffective utilization of DVB asa crosslinker and thus in the incomplete development of the crosslinkednetwork. In summary, Bicerano, et al. (1996), showed that the T_(g) oftypical “as-polymerized” S-DVB copolymers, as measured by the first DSCscan, increased only slowly with increasing w_(DVB), and furthermorethat the rate of further increase of T_(g) slowed down drastically forw_(DVB)>0.08. By contrast, in the second DSC scan (performed on S-DVBspecimens whose curing had been driven much closer to completion as aresult of the temperature ramp that had been applied during the firstscan), T_(g) grew much more rapidly with w_(DVB) over the entire rangeof up to w_(DVB)=0.2458 that was studied. The more extensively curedsamples resulting from the thermal history imposed by the first DSC scancan, thus, be considered to provide much closer approximations to theideal theoretical limit of a “fully cured” polymer network.

2. Effects of Heat Treatment on Key Properties of Thermoset Polymers

a. Maximum Possible Use Temperature

As was illustrated by Bicerano, et al. (1996) for S-DVB copolymers withw_(DVB) of up to 0.2458, enhancing the state of cure of a thermosetpolymer network can increase T_(g) very significantly relative to theT_(g) of the “as-polymerized” material. In practice, the heat distortiontemperature (HDT) is used most often as a practical indicator of thesoftening temperature of a polymer under load. As was shown by Takemori(1979), a systematic understanding of the HDT is possible through itsdirect correlation with the temperature dependences of the tensile (orequivalently, compressive) and shear elastic moduli. For amorphouspolymers, the precipitous decrease of these elastic moduli as T_(g) isapproached from below renders the HDT well-defined, reproducible, andpredictable. HDT is thus closely related to (and usually slightly lowerthan) T_(g) for amorphous polymers, so that it can be increasedsignificantly by increasing T_(g) significantly.

The HDT decreases gradually with increasing magnitude of the load usedin its measurement. For example, for general-purpose polystyrene (whichhas T_(g)=100° C.), HDT=95° C. under a load of 0.46 MPa and HDT=85° C.under a load of 1.82 MPa are typical values. However, the compressiveloads deep in an oil well or natural gas well are normally far higherthan the standard loads (0.46 MPa and 1.82 MPa) used in measuring theHDT. Consequently, amorphous thermoset polymer particles can be expectedto begin to deform significantly at a lower temperature than the HDT ofthe polymer measured under the standard high load of 1.82 MPa. Thisdeformation will cause a decrease in the conductivities of liquids andgases through the propped fracture, and hence in the loss ofeffectiveness as a proppant, at a somewhat lower temperature than theHDT value of the polymer measured under the standard load of 1.82 MPa.

b. Mechanical Properties

As was discussed earlier, Nishimori, et. al. (JP1992-22230) used heattreatment to increase the compressive elastic modulus of their S-DVBparticles (intended for use in liquid crystal display panels)significantly at room temperature (and hence far below T_(g)).Deformability under a compressive load is inversely proportional to thecompressive elastic modulus. It is, therefore, important to considerwhether one may also anticipate major benefits from heat treatment interms of the reduction of the deformability of thermoset polymerparticles intended for oil and natural gas drilling applications, whenthese particles are used in subterranean environments where thetemperature is far below the T_(g) of the particles. As explained below,the enhancement of curing via post-polymerization heat treatment isgenerally expected to have a smaller effect on the compressive elasticmodulus (and hence on the proppant performance) of thermoset polymerparticles when used in oil and natural gas drilling applications attemperatures far below their T_(g).

Nishimori, et. al. (JP1992-22230) used very large amounts of DVB(w_(DVB)>>0.2). By contrast, much smaller amounts of DVB (w_(DVB)≦0.2)must be used for economic reasons in the “lower value” oil and naturalgas drilling applications. The elastic moduli of a polymer attemperatures far below T_(g) are determined primarily by deformationsthat are of a rather local nature and hence on a short length scale.Some enhancement of the crosslink density via further curing (when thenetwork junctions created by the crosslinks are far away from each otherto begin with) will hence not normally have nearly as large an effect onthe elastic moduli as when the network junctions are very close to eachother to begin with and then are brought even closer by the enhancementof curing via heat treatment. Consequently, while the compressiveelastic modulus can be expected to increase significantly upon heattreatment when w_(DVB) is very large, any such effect will normally beless pronounced at low values of w_(DVB). In summary, it can thusgenerally be expected that the enhancement of the compressive elasticmodulus at temperatures far below T_(g) will probably be small for thetypes of formulations that are most likely to be used in the synthesisof thermoset polymer particles for oil and natural gas drillingapplications.

3. Effects of Nanoparticle Incorporation on Key Properties of ThermosetPolymers

a. Maximum Possible Use Temperature

As was pointed out by Takemori (1979), the addition of rigid fillers hasa negligible effect on the HDT of amorphous polymers. However,nanocomposite materials and technologies had not yet been developed in1979. It is, hence, important to consider, based on the data that havebeen gathered and the insights that have been obtained more recently,whether nanofillers may be expected to behave in a qualitativelydifferent manner because of their geometric characteristics.

A review article by Aharoni (1998) considered this question and showedthat three criteria must be considered. Here are the most relevantexcerpts from his article: “When a combination of the following threeconditions is fulfilled, then the glass transition temperature . . . maybe increased relative to that of the same polymer in the absence ofthese three conditions . . . . First, very large surface area of a rigidheterogeneous material in close contact with the amorphous phase of thepolymer. Such large surface areas may be obtained by having a rigidadditive material extremely finely ground, preferably to nanometerlength scale. Second, strong attractive interactions should existbetween the heterogeneous surfaces and the polymer. In the absence ofstrong attractive interactions with the heterogeneous rigid surfaces,the chain segments in the boundary layer are capable of relaxing to astate approximating the bulk polymer and the T_(g) will be identical orvery slightly higher than that of the pure bulk polymer. Third, measureof motional cooperation must exist between interchain and intrachainfragments. Unlike the effects of high modulus heterogeneous additives onthe averaged modulus of the system in which they are present, theelevation of T_(g) of the polymer matrix was repeatedly shown to requirenot only that the polymer itself will be a high molecular weightsubstance, but that the additive will be finely comminuted to generatevery large polymer-heterophase interfacial surface area, and, especiallyimportant, that strong attractive interactions will exist between thepolymer and the foreign additive. These interactions are generally of anionic, hydrogen bonding, or dipolar nature and, as a rule, require thatthe foreign additive will have surface energy higher than or at leastequal to, but never lower than, that of the amorphous polymer in whichit is being incorporated.”

Almost by definition, Aharoni's first condition will be satisfied forany nanofiller that has been dispersed well in the polymer matrix.Furthermore, since a thermoset polymer contains a covalently bondedthree-dimensional network structure, his third condition will also besatisfied if any thermoset polymer is used as the matrix material.However, in most systems, there will not be strong attractiveinteractions “generally of an ionic, hydrogen bonding, or dipolarnature” between the polymer and the nanofiller, so that the secondcriterion will not be satisfied. It can, therefore, be concluded that,for most combinations of polymer and nanofiller, T_(g) will not increasesignificantly upon incorporation of the nanofiller so that the maximumpossible use temperature will not increase significantly either. Therewill, however, be exceptions to this general rule. Combinations ofpolymer and nanofiller that manifest strong attractive interactions canbe found, and for such combinations both T_(g) and the maximum possibleuse temperature can increase significantly upon nanofillerincorporation.

b. Mechanical Properties

It is well-established that the incorporation of rigid fillers into apolymer matrix can produce a composite material which has significantlygreater stiffness (elastic modulus) and strength (stress required toinduce failure) than the base polymer. It is also well-established thatrigid nanofillers can generally stiffen and strengthen a polymer matrixmore effectively than conventional rigid fillers of similar compositionsince their geometries allow them to span (or “percolate through”) apolymer specimen at much lower volume fractions than conventionalfillers. This particular advantage of nanofillers over conventionalfillers is well-established and a major driving force for the vastresearch and development effort worldwide to develop new nanocompositeproducts.

FIG. 2 provides an idealized schematic illustration of the effectivenessof nanofillers in terms of their ability to “percolate through” apolymer specimen even when they are present at a low volume fraction. Itis important to emphasize that FIG. 2 is of a completely generic nature.It is presented merely to facilitate the understanding of nanofillerpercolation, without implying that it provides an accurate depiction ofthe expected behavior of any particular nanofiller in any particularpolymer matrix. In practice, the techniques of electron microscopy aregenerally used to observe the morphologies of actual embodiments of thenanocomposite concept. Specific examples of the ability of nanofillerssuch as carbon black and fumed silica to “percolate” at extremely lowvolume fractions when dispersed in polymers are provided by Zhang, et al(2001). The vast literature and trends on the dependences of percolationthresholds and packing fractions on particle shape, aggregation, andother factors, are reviewed by Bicerano, et al. (1999).

As has also been studied extensively [for example, see Okamoto, et al.(1999)] but is less widely recognized by workers in the field, theincorporation of rigid fillers of appropriate types and dimensions inthe right amount (often just a very small volume fraction) can toughen apolymer in addition to stiffening it and strengthening it. “Toughening”implies a reduction in the tendency to undergo brittle fracture. If andwhen it is realized for proppant particles, it is an importantadditional benefit since it reduces the risk of the generation of“fines” during use.

4. Technical Challenges to Nanoparticle Incorporation in ThermosetPolymers

It is important to also review the many serious technical challengesthat exist to the successful incorporation of nanoparticles in thermosetpolymers. Appreciation of these obstacles can help workers in the fieldof the invention gain a better understanding of the invention. There arethree major types of potential obstacles. In general, each potentialobstacle will tend to become more serious with increasing nanofillervolume fraction, so that it is usually easier to incorporate a smallvolume fraction of a nanofiller into a polymer than it is to incorporatea larger volume fraction. This subsection is subdivided further into thefollowing three subsections where each type of major potential obstaclewill be discussed in turn.

a. Difficulty of Dispersing Nanofiller

The most common difficulty that is encountered in preparing polymernanocomposites involves the need to disperse the nanofiller. Thespecific details of the source and severity of the difficulty, and ofthe methods that may help overcome the difficulty, differ between typesof nanofillers, polymers, and fabrication processes (for example, the“in situ” synthesis of the polymer in an aqueous or organic mediumcontaining the nanofiller, versus the addition of the nanofiller into amolten polymer). However, some important common aspects can beidentified.

Most importantly, nanofiller particles of the same kind often havestrong attractive interactions with each other. As a result, they tendto “clump together”; for example, preferably into agglomerates (if thenanofiller is particulate), bundles (if the nanofiller is fibrous), orstacks (if the nanofiller is discoidal). In most systems, theirattractive interactions with each other are stronger than theirinteractions with the molecules constituting the dispersing medium, sothat their dispersion is thermodynamically disfavored and henceextremely difficult.

Even in systems where the dispersion of the nanofillers isthermodynamically favored, it is often still very difficult to achievebecause of the large kinetic barriers (activation energies) that must besurmounted. Consequently, nanofillers are very rarely easy to dispersein a polymer.

b. High Dispersion Viscosity

Another difficulty with the fabrication of nanocomposites is the factthat, once the nanofiller is dispersed in the appropriate medium (forexample, an aqueous or organic medium containing the nanofiller for the“in situ” synthesis of the polymer, or a molten polymer into whichnanofiller is added), the viscosity of the resulting dispersion may (andoften does) become very high. When this happens, it can impede thesuccessful execution of the fabrication process steps that must followthe dispersion of the nanofiller to complete the preparation of thenanocomposite.

Dispersion rheology is a vast area of both fundamental and appliedresearch. It dates back to the 19^(th) century, so that there is a vastcollection of data and a good fundamental understanding of the factorscontrolling the viscosities of dispersions. Nonetheless, it is still atthe frontiers of materials science, so that major new experimental andtheoretical progress is continuing to be made. In fact, the advent ofnanotechnology, and the frequent emergence of high dispersion viscosityas an obstacle to the fabrication of polymer nanocomposites, have beeninstrumental in advancing the state of the art in this field. Bicerano,et al. (1999) have provided a comprehensive overview which can serve asa resource for workers interested in learning more about this topic.

c. Interference with Polymerization and Network Formation

An additional potential difficulty may be encountered in systems wherechemical reactions are taking place in a medium containing a nanofiller.This is the possibility that the nanofiller may have an adverse effecton the chemical reactions. As can reasonably be expected, any suchadverse effects can be far more severe in systems where polymerizationand network formation take place simultaneously in the presence of ananofiller than they can in systems where preformed polymer chains arecrosslinked in the presence of a nanofiller. The preparation of an S-DVBnanocomposite via suspension polymerization in a medium containing ananofiller is an example of a process where polymerization and networkformation both take place in the presence of a nanofiller. On the otherhand, the vulcanization of a nanofilled rubber is a process wherepreformed polymer chains are crosslinked in the presence of ananofiller.

The combined consideration of the work of Lipatov, et al. (1966, 1968),Popov, et al. (1982), and Bryk, et al. (1985, 1986, 1988) helps inproviding a broad perspective into the nature of the difficulties thatmay arise. To summarize, the presence of a filler with a high specificsurface area can disrupt both polymerization and network formation in aprocess such as the suspension polymerization of an S-DVB copolymernanocomposite. These outcomes can arise from the combined effects of theadsorption of initiators on the surfaces of the nanofiller particles andthe interactions of the growing polymer chains with the nanofillersurfaces. Adsorption on the nanofiller surface can affect the rate ofthermal decomposition of the initiator. Interactions of the growingpolymer chains with the nanofiller surfaces can result both in thereduction of the mobility of growing polymer chains and in theirbreakage. Very strong attractions between the initiator and thenanofiller surfaces (for example, the grafting of the initiators on thenanofiller surfaces) can potentially augment all of these detrimentaleffects.

Taguchi, et al. (1999) provided a fascinating example of how drasticallythe formulation can affect the particle morphology. They described theresults obtained by adding hydrophilic fine powders [nickel (Ni) of meanparticle size 0.3 microns, indium oxide (In₂O₃) of mean particle size0.03 microns, and magnetite (Fe₃O₄) of mean particle size 0.1, 0.3 or0.9 microns] to the aqueous phase during the suspension polymerizationof S-DVB. These particles had such a strong affinity to the aqueousphase that they did not even go inside the S-DVB beads. Instead, theyremained entirely outside the beads. Consequently, the compositeparticles consisted of S-DVB beads whose surfaces were uniformly coveredby a coating of inorganic powder. Furthermore, these S-DVB beads rapidlybecame smaller with increasing amount of powder at a fixed powderparticle diameter, as well as with decreasing powder particle diameter(and hence increasing number concentration of powder particles) at agiven powder weight fraction.

SUMMARY OF THE INVENTION

The present invention involves a novel approach towards the practicaldevelopment of stiff, strong, tough, heat resistant, and environmentallyresistant ultralightweight particles, for use in the construction,drilling, completion and/or fracture stimulation of oil and natural gaswells.

The disclosure is summarized below in three key aspects: (A)Compositions of Matter (thermoset nanocomposite particles that exhibitimproved properties compared with prior art), (B) Processes (methods formanufacture of said compositions of matter), and (C) Applications(utilization of said compositions of matter in the construction,drilling, completion and/or fracture stimulation of oil and natural gaswells).

The disclosure describes lightweight thermoset nanocomposite particleswhose properties are improved relative to prior art. The particlestargeted for development include, but are not limited to, terpolymers ofstyrene, ethyvinylbenzene and divinylbenzene; reinforced by particulatecarbon black of nanoscale dimensions. The particles exhibit any one orany combination of the following properties: enhanced stiffness,strength, heat resistance, and/or resistance to aggressive environments;and/or improved retention of high conductivity of liquids and/or gasesthrough packings of said particles when said packings are placed inpotentially aggressive environments under high compressive loads atelevated temperatures.

The disclosure also describes processes that can be used to manufacturesaid particles. The fabrication processes targeted for developmentinclude, but are not limited to, suspension polymerization in thepresence of nanofiller, and optionally post-polymerization heattreatment with said particles still in the reactor fluid that remainsafter the suspension polymerization to further advance the curing of thematrix polymer.

The disclosure finally describes the use of said particles in practicalapplications. The targeted applications include, but are not limited to,the construction, drilling, completion and/or fracture stimulation ofoil and natural gas wells; for example, as a proppant partial monolayer,a proppant pack, an integral component of a gravel pack completion, aball bearing, a solid lubricant, a drilling mud constituent, and/or acement additive.

A. Compositions of Matter

The compositions of matter of the present invention are thermosetpolymer nanocomposite particles where one or optionally more than onetype of nanofiller is intimately embedded in a polymer matrix. Anyadditional formulation component(s) familiar to those skilled in the artcan also be used during the preparation of said particles; such asinitiators, catalysts, inhibitors, dispersants, stabilizers, rheologymodifiers, buffers, antioxidants, defoamers, impact modifiers,plasticizers, pigments, flame retardants, smoke retardants, or mixturesthereof. Some of the said additional component(s) may also become eitherpartially or completely incorporated into said particles in someembodiments of the invention. However, the two required major componentsof said particles are a thermoset polymer matrix and at least onenanofiller. Hence this subsection will be further subdivided into threesubsections. Its first subsection will teach the volume fraction ofnanofiller(s) that may be used in the particles of the invention. Itssecond subsection will teach the types of thermoset polymers that may beused as matrix materials. Its third subsection will teach the types ofnanofillers that may be incorporated.

1. Nanofiller Volume Fraction

By definition, a nanofiller possesses at least one principal axisdimension whose length is less than 0.5 microns (500 nanometers). Thisgeometric attribute is what differentiates a nanofiller from a finelydivided conventional filler, such as the fillers taught by McDaniel, etal. (U.S. Pat. No. 6,632,527) whose characteristic lengths ranged from0.5 microns to 60 microns.

The dispersion of a nanofiller in a polymer is generally more difficultthan the dispersion of a conventional filler of similar chemicalcomposition in the same polymer. However, if dispersed properly duringcomposite particle fabrication, nanofillers can reinforce the matrixpolymer far more efficiently than conventional fillers. Consequently,while 60% to 90% by volume of filler is claimed by McDaniel, et al.(U.S. Pat. No. 6,632,527), only 0.001% to 60% by volume of nanofiller isclaimed in the present invention.

Without reducing the generality of the present invention, a nanofillervolume fraction of 0.1% to 15% is used in its currently preferredembodiments.

2. Matrix Polymers

Any rigid thermoset polymer may be used as the matrix polymer of thepresent invention.

Rigid thermoset polymers are, in general, amorphous polymers wherecovalent crosslinks provide a three-dimensional network. However, unlikethermoset elastomers (often referred to as “rubbers”) which also possessa three-dimensional network of covalent crosslinks, the rigid thermosetsare, by definition, “stiff”. In other words, they have high elasticmoduli at “room temperature” (25° C.), and often up to much highertemperatures, because their combinations of chain segment stiffness andcrosslink density result in a high glass transition temperature.

Some examples of rigid thermoset polymers that can be used as matrixmaterials of the invention will be provided below. It is to beunderstood that these examples are being provided without reducing thegenerality of the invention, merely to facilitate the teaching of theinvention.

Rigid thermoset polymers that are often used as matrix (often referredto as “binder”) materials in composites include, but are not limited to,crosslinked epoxies, epoxy vinyl esters, polyesters, phenolics,polyurethanes, and polyureas. Rigid thermoset polymers that are usedless often because of their high cost despite their exceptionalperformance include, but are not limited to, crosslinked polyimides.These various types of polymers can, in different embodiments of theinvention, be prepared by starting either from their monomers, or fromoligomers that are often referred to as “prepolymers”, or from suitablemixtures of monomers and oligomers.

Many additional types of rigid thermoset polymers can also be used asmatrix materials in composites, and are all within the scope of theinvention. Such polymers include, but are not limited to, variousfamilies of crosslinked copolymers prepared most often by thepolymerization of vinylic monomers, of vinylidene monomers, or ofmixtures thereof.

The “vinyl fragment” is commonly defined as the CH₂═CH— fragment. So a“vinylic monomer” is a monomer of the general structure CH₂═CHR where Rcan be any one of a vast variety of molecular fragments or atoms (otherthan hydrogen). When a vinylic monomer CH₂═CHR reacts, it isincorporated into the polymer as the —CH₂—CHR— repeat unit. Among rigidthermosets built from vinylic monomers, the crosslinked styrenics andcrosslinked acrylics are especially familiar to workers in the field.Some other familiar types of vinylic monomers (among others) include theolefins, vinyl alcohols, vinyl esters, and vinyl halides.

The “vinylidene fragment” is commonly defined as the CH₂═CR″— fragment.So a “vinylidene monomer” is a monomer of the general structureCH₂═CR′R″ where R′ and R″ can each be any one of a vast variety ofmolecular fragments or atoms (other than hydrogen). When a vinylidenemonomer CH₂═CR′R″ reacts, it is incorporated into a polymer as the—CH₂—CR′R″-repeat unit. Among rigid thermosets built from vinylidenepolymers, the crosslinked alkyl acrylics [such as crosslinkedpoly(methyl methacrylate)] are especially familiar to workers in thefield. However, vinylidene monomers similar to each type of vinylmonomer (such as the styrenics, acrylates, olefins, vinyl alcohols,vinyl esters and vinyl halides, among others) can be prepared. Oneexample of particular interest in the context of styrenic monomers isα-methyl styrene, a vinylidene-type monomer that differs from styrene (avinyl-type monomer) by having a methyl (—CH₃) group serving as the R″fragment replacing the hydrogen atom attached to the α-carbon.

Thermosets based on vinylic monomers, on vinylidene monomers, or onmixtures thereof, are typically prepared by the reaction of a mixturecontaining one or more non-crosslinking (difunctional) monomer and oneor more crosslinking (three or higher functional) monomers. Allvariations in the choices of the non-crosslinking monomer(s), thecrosslinking monomers(s), and their relative amounts [subject solely tothe limitation that the quantity of the crosslinking monomer(s) must notbe less than 1% by weight], are within the scope of the invention.

Without reducing the generality of the invention, in its currentlypreferred embodiments, the thermoset matrix consists of a terpolymer ofstyrene (non-crosslinking), ethyvinylbenzene (also non-crosslinking),and divinylbenzene (crosslinking), with the weight fraction ofdivinylbenzene ranging from 3% to 35% by weight of the starting monomermixture.

3. Nanofillers

By definition, a nanofiller possesses at least one principal axisdimension whose length is less than 0.5 microns (500 nanometers). Somenanofillers possess only one principal axis dimension whose length isless than 0.5 microns. Other nanofillers possess two principal axisdimensions whose lengths are less than 0.5 microns. Yet othernanofillers possess all three principal axis dimensions whose lengthsare less than 0.5 microns. Any reinforcing material possessing onenanoscale dimension, two nanoscale dimensions, or three nanoscaledimensions, can be used as the nanofiller in embodiments of theinvention. Any mixture of two or more different types of suchreinforcing materials can also be used as the nanofiller in embodimentsof the invention.

Some examples of nanofillers that can be incorporated into thenanocomposites of the invention will be provided below. It is to beunderstood that these examples are being provided without reducing thegenerality of the invention, merely to facilitate the teaching of theinvention.

Nanoscale carbon black, fumed silica and fumed alumina, such as productsof these types that are currently being manufactured by the CabotCorporation, consist of aggregates of small primary particles. See FIG.3 for a schematic illustration of such an aggregate, and of a largeragglomerate. The aggregates may contain many very small primaryparticles, often arranged in a “fractal” pattern, resulting in aggregateprincipal axis dimensions that are also shorter than 0.5 microns. Theseaggregates (and not the individual primary particles that constitutethem) are, in general, the smallest units of these nanofillers that aredispersed in a polymer matrix under normal fabrication conditions. Theavailable grades of such nanofillers include variations in specificsurface area, extent of branching (structure) in the aggregates, andchemical modifications intended to facilitate dispersion in differenttypes of media (such as aqueous or organic mixtures). Some product typesof such nanofillers are also provided in “fluffy” grades of lower bulkdensity that are easier to disperse than the base grade but lessconvenient to transport and store since the same weight of materialoccupies more volume when it is in its fluffy form. Some products gradesof such nanofillers are also provided pre-dispersed in an aqueousmedium.

Carbon nanotubes, carbon nanofibers, and cellulosic nanofibersconstitute three other classes of nanofillers. When separated from eachother by breaking up the bundles in which they are often found and thendispersed well in a polymer, they serve as fibrous reinforcing agents.In different products grades, they may have two principal axisdimensions in the nanoscale range (below 500 nanometers), or they mayhave all three principal axis dimensions in the nanoscale range (if theyhave been prepared by a process that leads to the formation of shorternanotubes or nanofibers). Currently, carbon nanotubes constitute themost expensive nanofillers of fibrous shape. Carbon nanotubes areavailable in single-wall and multi-wall versions. The single-wallversions offer the highest performance, but currently do so at a muchhigher cost than the multi-wall versions. Nanotubes prepared frominorganic materials (such as boron nitride) are also available.

Natural and synthetic nanoclays constitute another major class ofnanofiller. Nanocor and Southern Clay Products are the two leadingsuppliers of nanoclays at this time. When “exfoliated” (separated fromeach other by breaking up the stacks in which they are normally found)and dispersed well in a polymer, the nanoclays serve as discoidal(platelet-shaped) reinforcing agents. The thickness of an individualplatelet is around one nanometer (0.001 microns). The lengths in theother two principal axis dimensions are much larger. They range between100 and 500 nanometers in many product grades, thus resulting in aplatelet-shaped nanofiller that has three nanoscale dimensions. Theyexceed 500 nanometers, and thus result in a nanofiller that has only onenanoscale dimension, in some other grades.

Many additional types of nanofillers are also available; including, butnot limited to, very finely divided grades of fly ash, the polyhedraloligomeric silsesquioxanes, and clusters of different types of metals,metal alloys, and metal oxides. Since the development of nanofillers isan area that is at the frontiers of materials research and development,the future emergence of yet additional types of nanofillers that are notcurrently known may also be readily anticipated.

Without reducing the generality of the invention, in its currentlypreferred embodiments, nanoscale carbon black grades supplied by CabotCorporation are being used as the nanofiller.

B. Processes

In most cases, the incorporation of a nanofiller into the thermosetpolymer matrix will increase the compressive elastic modulus uniformlythroughout the entire use temperature range (albeit usually not byexactly the same factor at each temperature), while not increasing T_(g)significantly. The resulting nanocomposite particles will then performbetter as proppants over their entire use temperature range, but withoutan increase in the maximum possible use temperature itself. On the otherhand, if a suitable post-polymerization process step is applied to thenanocomposite particles, in many cases the curing reaction will bedriven further towards completion so that T_(g) (and hence also themaximum possible use temperature) will increase along with the increaseinduced by the nanofiller in the compressive elastic modulus.

Processes that may be used to enhance the degree of curing of athermoset polymer include, but are not limited to, heat treatment (whichmay be combined with stirring and/or sonication to enhance itseffectiveness), electron beam irradiation, and ultraviolet irradiation.We focused mainly on the use of heat treatment in order to increase theT_(g) of the thermoset matrix polymer, to make it possible to usenanofiller incorporation and post-polymerization heat treatment ascomplementary methods, to improve the performance characteristics of theparticles even further by combining the anticipated main benefits ofeach method. FIG. 4 provides an idealized schematic illustration of thebenefits of implementing these methods and concepts.

The processes that may be used for the fabrication of the thermosetnanocomposite particles of the invention have at least one, andoptionally two, major step(s). The required step is the formation ofsaid particles by means of a process that allows the intimate embedmentof the nanofiller in the polymer matrix. The optional step is the use ofan appropriate postcuring method to advance the curing reaction of thethermoset matrix and to thus obtain a polymer network that approachesthe “fully cured” limit. Consequently, this subsection will be furthersubdivided into two subsections, dealing with polymerization and withpostcure respectively.

1. Polymerization and Network Formation in Presence of Nanofiller

Any method for the fabrication of thermoset composite particles known tothose skilled in the art may be used to prepare embodiments of thethermoset nanocomposite particles of the invention. Without reducing thegenerality of the invention, some such methods will be discussed belowto facilitate the teaching of the invention.

The most practical methods for the formation of composites containingrigid thermoset matrix polymers involve the dispersion of the filler ina liquid (aqueous or organic) medium followed by the “in situ” formationof the crosslinked polymer network around the filler. This is incontrast with the formation of thermoplastic composites where meltblending can instead also be used to mix a filler with a fully formedmolten polymer. It is also in contrast with the vulcanization of afilled rubber, where preformed polymer chains are crosslinked in thepresence of a filler.

The implementation of such methods in the preparation of thermosetnanocomposite particles is usually more difficult to accomplish inpractice than their implementation in the preparation of compositeparticles containing conventional fillers. As discussed earlier, commonchallenges involve difficulties in dispersing the nanofiller, highnanofiller dispersion viscosity, and possible interferences of thenanofiller with polymerization and network formation. Nonetheless, thesechallenges can all be surmounted by making judicious choices of theformulation ingredients and their proportions, and then also determiningand using the optimum processing conditions.

McDaniel, et al. (U.S. Pat. No. 6,632,527) prepared polymer compositeparticles with thermoset matrix formulations. Their formulations werebased on at least one member of the group consisting of inorganicbinder, epoxy resin, novolac resin, resole resin, polyurethane resin,alkaline phenolic resole curable with ester, melamine resin,urea-aldehyde resin, urea-phenol-aldehyde resin, furans, syntheticrubber, and/or polyester resin. They taught the incorporation ofconventional filler particles, whose sizes ranged from 0.5 microns to 60microns, at 60% to 90% by volume. Their fabrication processes differedin details depending on the specific formulation, but in generalincluded steps involving the mixing of a binder stream with a fillerparticle stream, agglomerative granulation, and the curing of agranulated material stream to obtain thermoset composite particles ofthe required size and shape. These processes can also be used to preparethe thermoset nanocomposite particles of the present invention, wherenanofillers possessing at least one principal axis dimension shorterthan 0.5 microns are used at a volume fraction that does not exceed 60%and that is far smaller than 60% in the currently preferred embodiments.The processes of McDaniel, et al. (U.S. Pat. No. 6,632,527) are, hence,incorporated herein by reference.

As was discussed earlier, many additional types of thermoset polymerscan also be used as the matrix materials in composites. Examples includecrosslinked polymers prepared from various styrenic, acrylic or olefinicmonomers (or mixtures thereof). It is more convenient to prepareparticles of such thermoset polymers (as well as of their composites andnanocomposites) by using methods that can produce said particlesdirectly in the desired (usually substantially spherical) shape duringpolymerization from the starting monomers. (While it is a goal of thisinvention to create spherical particles, it is understood that it isexceedingly difficult as well as unnecessary to obtain perfectlyspherical particles. Therefore, particles with minor deviations from aperfectly spherical shape are considered perfectly spherical for thepurposes of this disclosure.) Suspension (droplet) polymerization is themost powerful method available for accomplishing this objective. Twomain approaches exist to suspension polymerization. The first approachis isothermal polymerization which is the conventional approach that hasbeen practiced for many decades. The second approach is “rapid ratepolymerization” as taught by Albright (U.S. Pat. No. 6,248,838) which isincorporated herein by reference. Without reducing the generality of theinvention, suspension polymerization as performed via the rapid ratepolymerization approach taught by Albright (U.S. Pat. No. 6,248,838) isused in the current preferred embodiments of the invention.

2. Optional Post-Polymerization Advancement of Curing and NetworkFormation

As was discussed earlier and illustrated in FIG. 1 with the data ofBicerano, et al. (1996), typical processes for the synthesis ofthermoset polymers may result in the formation of incompletely curednetworks, and may hence produce thermosets with lower glass transitiontemperatures and lower maximum use temperatures than is achievable withthe chosen formulation of reactants. Furthermore, difficulties relatedto incomplete cure may sometimes be exacerbated in thermosetnanocomposites because of the possibility of interference by thenanofiller in polymerization and network formation. Consequently, theuse of an optional post-polymerization process step (or a sequence ofsuch process steps) to advance the curing of the thermoset matrix of aparticle of the invention is an aspect of the invention. Suitablemethods include, but are not limited to, heat treatment (also known as“annealing”), electron beam irradiation, and ultraviolet irradiation.

Post-polymerization heat treatment is a very powerful method forimproving the properties and performance of S-DVB copolymers (as well asof many other types of thermoset polymers) by helping the polymernetwork approach its “full cure” limit. It is, in fact, the most easilyimplementable method for advancing the state of cure of S-DVB copolymerparticles. However, it is important to recognize that anotherpost-polymerization method (such as electron beam irradiation orultraviolet irradiation) may be the most readily implementable one foradvancing the state of cure of some other type of thermoset polymer. Theuse of any suitable method for advancing the curing of the thermosetpolymer that is being used as the matrix of a nanocomposite of thepresent invention after polymerization is within the scope of theinvention.

Without reducing the generality of the invention, among the suitablemethods, heat treatment is used as the optional post-polymerizationmethod to enhance the curing of the thermoset polymer matrix in thepreferred embodiments of the invention. Any desired thermal history canbe optionally imposed; such as, but not limited to, isothermal annealingat a fixed temperature; nonisothermal heat exposure with either acontinuous or a step function temperature ramp; or any combination ofcontinuous temperature ramps, step function temperature ramps, and/orperiods of isothermal annealing at fixed temperatures. In practice,while there is great flexibility in the choice of a thermal history, itmust be selected carefully to drive the curing reaction to the maximumfinal extent possible without inducing unacceptable levels of thermaldegradation.

Any significant increase in T_(g) by means of improved curing willtranslate directly into an increase of comparable magnitude in thepractical softening temperature of the polymer particles under thecompressive load imposed by the subterranean environment. Consequently,a significant increase of the maximum possible use temperature of thethermoset polymer particles is the most common benefit of advancing theextent of curing by heat treatment.

A practical concern during the imposition of optional heat treatment isrelated to the amount of material that is being subjected to heattreatment simultaneously. For example, very small amounts of materialcan be heat treated uniformly and effectively in vacuum; or in any inert(non-oxidizing) gaseous medium, such as, but not limited to, a helium ornitrogen “blanket”. However, heat transfer in a gaseous medium is notnearly as effective as heat transfer in an appropriately selected liquidmedium. Consequently, during the optional heat treatment of largequantities of the particles of the invention (such as, but not limitedto, the output of a run of a commercial-scale batch production reactor),it is usually necessary to use a liquid medium, and furthermore also tostir the particles vigorously to ensure that the heat treatment isapplied as uniformly as possible. Serious quality problems may arise ifheat treatment is not applied uniformly; for example, as a result of theparticles that were initially near the heat source being overexposed toheat and thus damaged, while the particles that were initially far awayfrom the heat source are not exposed to sufficient heat and are thus notsufficiently postcured.

If a gaseous or a liquid heat treatment medium is used, said medium maycontain, without limitation, one or a mixture of any number of types ofconstituents of different molecular structure. However, in practice,said medium must be selected carefully to ensure that its molecules willnot react with the crosslinked polymer particles to a sufficient extentto cause significant oxidative and/or other types of chemicaldegradation. In this context, it must also be kept in mind that manytypes of molecules which do not react with a polymer at ambienttemperature may react strongly with said polymer at elevatedtemperatures. The most relevant example in the present context is thatoxygen itself does not react with S-DVB copolymers at room temperature,while it causes severe oxidative degradation of S-DVB copolymers atelevated temperatures where there would not be much thermal degradationin its absence.

Furthermore, in considering the choice of medium for heat treatment, itis also important to keep in mind that organic molecules can swellorganic polymers, potentially causing “plasticization” and thusresulting in undesirable reductions of T_(g) and of the maximum possibleuse temperature. The magnitude of any such detrimental effect increaseswith increasing similarity between the chemical structures of themolecules in the heat treatment medium and of the polymer chains. Forexample, a heat transfer fluid consisting of aromatic molecules willtend to swell a styrene-divinylbenzene copolymer particle, as well astending to swell a nanocomposite particle containing such a copolymer asits matrix. The magnitude of this detrimental effect will increase withdecreasing relative amount of the crosslinking monomer (divinylbenzene)used in the formulation. For example, a styrene-divinylbenzene copolymerprepared from a formulation containing only 3% by weight ofdivinylbenzene will be far more susceptible to swelling in an aromaticliquid than a copolymer prepared from a formulation containing 35%divinylbenzene.

Various means known to those skilled in the art, including but notlimited to the stirring and/or the sonication of an assembly ofparticles being subjected to heat treatment, may also be optionally usedto enhance further the effectiveness of the optional heat treatment. Therate of thermal equilibration under a given thermal gradient, possiblycombined with the application of any such additional means, depends onmany factors. These factors include, but are not limited to, the amountof polymer particles being heat treated simultaneously, the shapes andcertain key physical and transport properties of these particles, theshape of the vessel being used for heat treatment, the medium being usedfor heat treatment, whether external disturbances (such as stirringand/or sonication) are being used to accelerate equilibration, and thedetails of the heat exposure schedule. Simulations based on the solutionof the heat transfer equations may hence be used optionally to optimizethe heat treatment equipment and/or the heat exposure schedule.

Without reducing the generality of the invention, in its currentlypreferred embodiments, the thermoset nanocomposite particles are left inthe reactor fluid that remains after suspension polymerization ifoptional heat treatment is to be used. Said reactor fluid thus serves asthe heat treatment medium; and simulations based on the solution of theheat transfer equations are used to optimize the heat exposure schedule.This embodiment of the optional heat treatment works especially well(without adverse effects such as degradation and/or swelling) inenhancing the curing of the thermoset matrix polymer in the currentlypreferred compositions of matter of the invention. Said preferredcompositions of matter consist of terpolymers of styrene,ethylvinylbenzene and divinylbenzene. Since the reactor fluid thatremains after the completion of suspension polymerization is aqueouswhile these terpolymers are very hydrophobic, the reactor fluid servesas an excellent heat transfer medium which does not swell the particles.The use of the reactor fluid as the medium for the optional heattreatment also has the advantage of simplicity since the particles wouldhave needed to be removed from the reactor fluid and placed in anotherfluid as an extra step before heat treatment if an alternative fluid hadbeen required.

It is, however, important to reemphasize the much broader scope of theinvention and the fact that the particular currently preferredembodiments summarized above constitute just a few among the vastvariety of possible qualitatively different classes of embodiments. Forexample, if a hydrophilic thermoset polymer particle were to bedeveloped as an alternative preferred embodiment of the invention infuture work, it would obviously not be possible to subject such anembodiment to heat treatment in an aqueous slurry, and a hydrophobicheat transfer fluid would work better for its optional heat treatment.

C. Applications

The obvious practical advantages [see a review by Edgeman (2004)] ofdeveloping the ability to use lightweight particles that possess almostneutral buoyancy relative to water have stimulated a considerable amountof work over the years. However, progress in this field of invention hasbeen very slow as a result of the many technical challenges that existto the successful development of cost-effective lightweight particlesthat possess sufficient stiffness, strength and heat resistance. Thepresent invention has resulted in the development of such stiff, strong,tough, heat resistant, and environmentally resistant ultralightweightparticles; and also of cost-effective processes for the fabrication ofsaid particles. As a result, a broad range of potential applications canbe envisioned and are being pursued for the use of the thermoset polymernanocomposite particles of the invention in the construction, drilling,completion and/or fracture stimulation of oil and natural gas wells.Without reducing the generality of the invention, in its currentlypreferred embodiments, the specific applications that are already beingevaluated are as a proppant partial monolayer, a proppant pack, anintegral component of a gravel pack completion, a ball bearing, a solidlubricant, a drilling mud constituent, and/or a cement additive.

It is also important to note that the current selection of preferredembodiments of the invention has resulted from our focus on applicationopportunities in the construction, drilling, completion and/or fracturestimulation of oil and natural gas wells. Many other applications canalso be envisioned for the compositions of matter that fall within thescope of thermoset nanocomposite particles of the invention. Forexample, one such application is described by Nishimori, et. al.(JP1992-22230), who developed heat-treated S-DVB copolymer (but notcomposite) particles prepared from formulations containing very high DVBweight fractions for use in liquid crystal display panels. Alternativeembodiments of the thermoset copolymer nanocomposite particles of thepresent invention, tailored towards the performance needs of thatapplication and benefiting from its less restrictive cost limitations,could potentially also be used in liquid crystal display panels.Considered from this perspective, it can be seen readily that thepotential applications of the particles of the invention extend farbeyond their uses by the oil and natural gas industry.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 shows the effects of advancing the curing reaction in a series ofisothermally polymerized styrene-divinylbenzene (S-DVB) copolymerscontaining different DVB weight fractions via heat treatment. Theresults of scans of S-DVB beads containing various weight fractions ofDVB (w_(DVB)), obtained by Differential Scanning calorimetry (DSC), andreported by Bicerano, et al. (1996), are compared. It is seen that theT_(g) of typical “as-polymerized” S-DVB copolymers, as measured by thefirst DSC scan, increased only slowly with increasing w_(DVB), andfurthermore that the rate of further increase of T_(g) slowed downdrastically for w_(DVB)>0.08. By contrast, in the second DSC scan(performed on S-DVB specimens whose curing had been driven much closerto completion as a result of the temperature ramp that had been appliedduring the first scan), T_(g) grew much more rapidly with w_(DVB) overthe entire range of up to W_(DVB)=0.2458 that was studied.

FIG. 2 provides an idealized, generic and schematic two-dimensionalillustration of how a very small volume fraction of a nanofiller may beable to “span” and thus “bridge through” a vast amount of space, thuspotentially enhancing the load bearing ability of the matrix polymersignificantly at much smaller volume fractions than possible withconventional fillers.

FIG. 3 illustrates the “aggregates” in which the “primary particles” ofnanofillers such as nanoscale carbon black, fumed silica and fumedalumina commonly occur. Such aggregates may contain many very smallprimary particles, often arranged in a “fractal” pattern, resulting inaggregate principal axis dimensions that are also shorter than 0.5microns. These aggregates (and not the individual primary particles thatconstitute them) are, usually, the smallest units of such nanofillersthat are dispersed in a polymer matrix under normal fabricationconditions, when the forces holding the aggregates together in the muchlarger “agglomerates” are overcome successfully. This illustration wasreproduced from the product literature of Cabot Corporation.

FIG. 4 provides an idealized schematic illustration, in the context ofthe resistance of thermoset polymer particles to compression as afunction of the temperature, of the most common benefits of using themethods of the present invention. In most cases, the densification ofthe crosslinked polymer network via post-polymerization heat treatmentwill have the main benefit of increasing the softening (and hence alsothe maximum possible use) temperature, along with improving theenvironmental resistance. On the other hand, in most cases, nanofillerincorporation will have the main benefits of increasing the stiffnessand strength. The use of nanofiller incorporation andpost-polymerization heat treatment together, as complementary methods,will thus often be able to provide all (or at least most) of thesebenefits simultaneously.

FIG. 5 provides a process flow diagram depicting the preparation of theexample. It contains four major blocks; depicting the preparation of theaqueous phase (Block A), the preparation of the organic phase (Block B),the mixing of these two phases followed by suspension polymerization(Block C), and the further process steps used after polymerization toobtain the “as-polymerized” and “heat-treated” samples of particles(Block D).

FIG. 6 shows the variation of the temperature with time duringpolymerization.

FIG. 7 shows the results of the measurement of the glass transitiontemperatures (T_(g)) of the three heat-treated thermoset nanocompositesamples via differential scanning calorimetry (DSC). The samples haveidentical compositions. They differ only as a result of the use ofdifferent heat treatment conditions after polymerization. T_(g) wasdefined as the temperature at which the curve showing the heat flow as afunction of the temperature goes through its inflection point.

FIG. 8 provides a schematic illustration of the configuration of theconductivity cell.

FIG. 9 shows the measured liquid conductivity of a packing of particlesof 14/16 U.S. mesh size (diameters ranging from 1.19 mm to 1.41 mm) fromSample 40m200C, at a coverage of 0.02 lb/ft², under a closure stress of4000 psi at a temperature of 190° F., as a function of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Because the invention will be understood better after further discussionof its currently preferred embodiments, further discussion of saidembodiments will now be provided. It is understood that said discussionis being provided without reducing the generality of the invention,since persons skilled in the art can readily imagine many additionalembodiments that fall within the full scope of the invention as taughtin the SUMMARY OF THE INVENTION section.

A. Nature, Attributes and Applications of Currently PreferredEmbodiments

The currently preferred embodiments of the invention are lightweightthermoset nanocomposite particles possessing high stiffness, strength,temperature resistance, and resistance to aggressive environments. Theseattributes, occurring in combination, make said particles especiallysuitable for use in many challenging applications in the construction,drilling, completion and/or fracture stimulation of oil and natural gaswells. Said applications include the use of said particles as a proppantpartial monolayer, a proppant pack, an integral component of a gravelpack completion, a ball bearing, a solid lubricant, a drilling mudconstituent, and/or a cement additive.

B. Thermoset Polymer Matrix

1. Constituents

The thermoset matrix in said particles consists of a terpolymer ofstyrene (S, non-crosslinking), ethyvinylbenzene (EVB, alsonon-crosslinking), and divinylbenzene (DVB, crosslinking) The preferencefor such a terpolymer instead of a copolymer of S and DVB is a result ofeconomic considerations. To summarize, DVB comes mixed with EVB in thestandard product grades of DVB, and the cost of DVB increases rapidlywith increasing purity in special grades of DVB. EVB is anon-crosslinking (difunctional) styrenic monomer. Its incorporation intothe thermoset matrix does not result in any significant changes in theproperties of the thermoset matrix or of nanocomposites containing saidmatrix, compared with the use of S as the sole non-crosslinking monomer.Consequently, it is far more cost-effective to use a standard (ratherthan purified) grade of DVB, thus resulting in a terpolymer where someof the repeat units originate from EVB.

2. Proportions

The amount of DVB in said terpolymer ranges from 3% to 35% by weight ofthe starting mixture of the three reactive monomers (S, EVB and DVB)because different applications require different maximum possible usetemperatures. Even when purchased in standard product grades where it ismixed with a large weight fraction of EVB, DVB is more expensive than S.It is, hence, useful to develop different product grades where themaximum possible use temperature increases with increasing weightfraction of DVB. Customers can then purchase the grades of saidparticles that meet their specific application needs as cost-effectivelyas possible.

C. Nanofiller

1. Constituents

The Monarch™ 280 product grade of nanoscale carbon black supplied byCabot Corporation is being used as the nanofiller in said particles. Thereason is that it has a relatively low specific surface area, highstructure, and a “fluffy” product form; rendering it especially easy todisperse.

2. Proportions

The use of too low a volume fraction of carbon black results inineffective reinforcement. The use of too high a volume fraction ofcarbon black may result in difficulties in dispersing the nanofiller,dispersion viscosities that are too high to allow further processingwith available equipment, and detrimental interference in polymerizationand network formation. The amount of carbon black ranges from 0.1% to15% by volume of said particles because different applications requiredifferent levels of reinforcement. Carbon black is more expensive thanthe monomers (S, EVB and DVB) currently being used in the synthesis ofthe thermoset matrix. It is, therefore, useful to develop differentproduct grades where the extent of reinforcement increases withincreasing volume fraction of carbon black. Customers can then purchasethe grades of said particles that meet their specific application needsas cost-effectively as possible.

D. Polymerization

Suspension polymerization is performed via rapid rate polymerization, astaught by Albright (U.S. Pat. No. 6,248,838) which is incorporatedherein by reference, for the fabrication of said particles. Rapid ratepolymerization has the advantage, relative to conventional isothermalpolymerization, of producing more physical entanglements in thermosetpolymers (in addition to the covalent crosslinks). Suspensionpolymerization involves the preparation of an the aqueous phase and anorganic phase prior to the commencement of the polymerization process.The Monarch™ 280 carbon black particles are dispersed in the organicphase prior to polymerization. The most important additional formulationcomponent (besides the reactive monomers and the nanofiller particles)that is used during polymerization is the initiator. The initiator mayconsist of one type molecule or a mixture of two or more types ofmolecules that have the ability to function as initiators. Additionalformulation components, such as catalysts, inhibitors, dispersants,stabilizers, rheology modifiers, buffers, antioxidants, defoamers,impact modifiers, plasticizers, pigments, flame retardants, smokeretardants, or mixtures thereof, may also be used when needed. Some ofthe additional formulation component(s) may become either partially orcompletely incorporated into the particles in some embodiments of theinvention.

E. Attainable Particle Sizes

Suspension polymerization produces substantially spherical polymerparticles. (While it is a goal of this invention to create sphericalparticles, it is understood that it is exceedingly difficult as well asunnecessary to obtain perfectly spherical particles. Therefore,particles with minor deviations from a perfectly spherical shape areconsidered perfectly spherical for the purposes of this disclosure.)Said particles can be varied in size by means of a number of mechanicaland/or chemical methods that are well-known and well-practiced in theart of suspension polymerization. Particle diameters attainable by suchmeans range from submicron values up to several millimeters. Hence saidparticles may be selectively manufactured over the entire range of sizesthat are of present interest and/or that may be of future interest forapplications in the oil and natural gas industry.

F. Optional Further Selection of Particles by Size

Optionally, after the completion of suspension polymerization, saidparticles can be separated into fractions having narrower diameterranges by means of methods (such as, but not limited to, sievingtechniques) that are well-known and well-practiced in the art ofparticle separations. Said narrower diameter ranges include, but are notlimited to, nearly monodisperse distributions. Optionally, assemblies ofparticles possessing bimodal or other types of special distributions, aswell as assemblies of particles whose diameter distributions followstatistical distributions such as gaussian or log-normal, can also beprepared.

The optional preparation of assemblies of particles having diameterdistributions of interest from any given “as polymerized” assembly ofparticles can be performed before or after any optional heat treatmentof said particles. Without reducing the generality of the invention, inthe currently most preferred embodiments of the invention, any optionalpreparation of assemblies of particles having diameter distributions ofinterest from the product of a run of the pilot plant or productionplant reactor is performed after the completion of any optional heattreatment of said particles.

The particle diameters of current practical interest for various uses inthe construction, drilling, completion and/or fracture stimulation ofoil and natural gas wells range from 0.1 to 4 millimeters. The specificdiameter distribution that would be most effective under givencircumstances depends on the details of the subterranean environment inaddition to depending on the type of application. The diameterdistribution that would be most effective under given circumstances maybe narrow or broad, monomodal or bimodal, and may also have otherspecial features (such as following a certain statistical distributionfunction) depending on both the details of the subterranean environmentand the type of application.

G. Optional Heat Treatment

Said particles are left in the reactor fluid that remains aftersuspension polymerization if optional heat treatment is to be used. Saidreactor fluid thus serves as the heat treatment medium. This approachworks especially well (without adverse effects such as degradationand/or swelling) in enhancing the curing of said particles where thepolymer matrix consists of a terpolymer of S, EVB and DVB. Since thereactor fluid that remains after the completion of suspensionpolymerization is aqueous while these terpolymers are very hydrophobic,the reactor fluid serves as an excellent heat transfer medium which doesnot swell the particles. The use of the reactor fluid as the medium forthe optional heat treatment also has the advantage of simplicity sincethe particles would have needed to be removed from the reactor fluid andplaced in another fluid as an extra step before heat treatment if analternative fluid had been required.

Detailed and realistic simulations based on the solution of the heattransfer equations are often used optionally to optimize the heatexposure schedule if optional heat treatment is to be used. It has beenfound that such simulations become increasingly useful with increasingquantity of particles that will be heat treated simultaneously. Thereason is the finite rate of heat transfer. Said finite rate results inslower and more difficult equilibration with increasing quantity ofparticles and hence makes it especially important to be able to predicthow to cure most of the particles further uniformly and sufficientlywithout overexposing many of the particles to heat.

EXAMPLE

The currently preferred embodiments of the invention will be understoodbetter in the context of a specific example. It is to be understood thatsaid example is being provided without reducing the generality of theinvention. Persons skilled in the art can readily imagine manyadditional examples that fall within the scope of the currentlypreferred embodiments as taught in the DETAILED DESCRIPTION OF THEINVENTION section. Persons skilled in the art can, furthermore, alsoreadily imagine many alternative embodiments that fall within the fullscope of the invention as taught in the SUMMARY OF THE INVENTIONsection.

A. Summary

The thermoset matrix was prepared from a formulation containing 10% DVBby weight of the starting monomer mixture. The DVB had been purchased asa mixture where only 63% by weight consisted of DVB. The actualpolymerizable monomer mixture used in preparing the thermoset matrixconsisted of roughly 84.365% S, 5.635% EVB and 10% DVB by weight.

Carbon black (Monarch 280) was incorporated into the particles, at 0.5%by weight, via dispersion in the organic phase of the formulation priorto polymerization. Since the specific gravity of carbon black is roughly1.8 while the specific gravity of the polymer is roughly 1.04, theamount of carbon black incorporated into the particles was roughly 0.29%by volume.

Suspension polymerization was performed in a pilot plant reactor, viarapid rate polymerization as taught by Albright (U.S. Pat. No.6,248,838) which is incorporated herein by reference. In applying thismethod, the “dual initiator” approach, wherein two initiators withdifferent thermal stabilities are used to help drive the reaction of DVBfurther towards completion, was utilized.

The required tests only require a small quantity of particles. The useof a liquid medium (such as the reactor fluid) is unnecessary for theheat treatment of a small sample. Roughly 500 grams of particles werehence removed from the slurry, washed, spread very thin on a tray,heat-treated for ten minutes at 200° C. in an oven in an inert gasenvironment, and submitted for testing.

The glass transition temperature of these “heat-treated” particles, andthe liquid conductivity of packings thereof, were then measured byindependent testing laboratories (Impact Analytical in Midland, Mich.,and FracTech Laboratories in Surrey, United Kingdom, respectively).

FIG. 5 provides a process flow diagram depicting the preparation of theexample. It contains four major blocks; depicting the preparation of theaqueous phase (Block A), the preparation of the organic phase (Block B),the mixing of these two phases followed by suspension polymerization(Block C), and the further process steps used after polymerization toobtain the “as-polymerized” and “heat-treated” samples of particles(Block D).

The following subsections will provide further details on theformulation, preparation and testing of this working example, to enablepersons who are skilled in the art to reproduce the example.

B. Formulation

An aqueous phase and an organic phase must be prepared prior tosuspension polymerization. The aqueous phase and the organic phase,which were prepared in separate beakers and then used in the suspensionpolymerization of the particles of this example, are described below.

1. Aqueous Phase

The aqueous phase used in the suspension polymerization of the particlesof this example, as well as the procedure used to prepare said aqueousphase, are summarized in TABLE 1. TABLE 1. The aqueous phase wasprepared by adding Natrosol Plus 330 and gelatin (Bloom strength 250) towater, heating to 65° C. to disperse the Natrosol Plus 330 and thegelatin in the water, and then adding sodium nitrite and sodiumcarbonate. Its composition is listed below.

INGREDIENT WEIGHT (g) % Water 1493.04 98.55 Natrosol Plus 330(hydroxyethylcellulose) 7.03 0.46 Gelatin (Bloom strength 250) 3.51 0.23Sodium Nitrite (NaNO₂) 4.39 0.29 Sodium Carbonate (Na₂CO₃) 7.03 0.46Total Weight in Grams 1515.00 100.00

2. Organic Phase

The organic phase used in the suspension polymerization of the particlesof this example, as well as the procedure used to prepare said organicphase, are summarized in TABLE 2. Note that the nanofiller (carbonblack) was added to the organic phase in this particular example. TABLE2. The organic phase was prepared by placing the monomers, benzoylperoxide (an initiator), t-amyl peroxy(2-ethylhexyl)monocarbonate (TAEC,also an initiator), Disperbyk-161 and carbon black together andagitating the resulting mixture for at least 15 minutes to dispersecarbon black in the mixture. Its composition is listed below. Aftertaking the other components of the 63% DVB mixture into account, thepolymerizable monomer mixture actually consisted of roughly 84.365% S,5.635% EVB and 10% DVB by weight. The total polymerizable monomer weightof was 1356.7 grams. The resulting thermoset nanocomposite particlesthus contained [100×6.8/(1356.7+6.8)]=0.5% by weight of carbon black.

INGREDIENT WEIGHT (g) % Styrene (pure) 1144.58 82.67 Divinylbenzene (63%DVB, 215.35 15.56 98.5% polymerizable monomers) Carbon black (Monarch280) 6.8 0.49 Benzoyl peroxide 13.567 0.98 t-Amyl 4.07 0.29peroxy(2-ethylhexyl)monocarbonate (TAEC) Disperbyk-161 0.068 0.0049Total Weight in Grams 1384.435 100C. Preparation of Particles from Formulation

Once the formulation is prepared, its aqueous and organic phases aremixed, polymerization is performed, and “as-polymerized” and“heat-treated” particles are obtained, as described below.

1. Mixing

The aqueous phase was added to the reactor at 65° C. The organic phasewas then introduced over roughly 5 minutes with agitation at the rate of90 rpm. The mixture was held at 65° C. with stirring at the rate of 90rpm for at least 15 minutes or until proper dispersion had taken placeas manifested by the equilibration of the droplet size distribution.

2. Polymerization

The temperature was ramped from 65° C. to 78° C. in 10 minutes. It wasthen further ramped from 78° C. to 90° C. at the rate of 0.1° C. perminute in 120 minutes. It was then held at 90° C. for 90 minutes toprovide most of the conversion of monomer to polymer, with benzoylperoxide (half life of one hour at 92° C.) as the effective initiator.It was then further ramped to 115° C. in 30 minutes and held at 115° C.for 180 minutes to advance the curing with TAEC (half life of one hourat 117° C.) as the effective initiator. The particles were thus obtainedin an aqueous slurry. FIG. 6 shows the variation of the temperature withtime during polymerization.

3. “As-Polymerized” Particles

The aqueous slurry was cooled to 40° C. It was then poured onto a 60mesh (250 micron) sieve to remove the aqueous reactor fluid as well asany undesirable small particles that may have formed duringpolymerization. The “as-polymerized” beads of larger than 250 microndiameter obtained in this manner were then washed three times with warm(40° C. to 50° C.) water

4. “Heat-Treated” Particles

Three sets of “heat-treated” particles, which were imposed to differentthermal histories during the post-polymerization heat treatment, wereprepared from the “as-polymerized” particles. In preparing each of theseheat-treated samples, washed beads were removed from the 60 mesh sieve,spread very thin on a tray, placed in an oven under an inert gas(nitrogen) blanket, and subjected to the desired heat exposure. Sample10m200C was prepared with isothermal annealing for 10 minutes at 200° C.Sample 40m200C was prepared with isothermal annealing for 40 minutes at200° C. to explore the effects of extending the duration of isothermalannealing at 200° C. Sample 10m220C was prepared with isothermalannealing for 10 minutes at 220° C. to explore the effects of increasingthe temperature at which isothermal annealing is performed for aduration of 10 minutes. In each case, the oven was heated to 100° C.,the sample was placed in the oven and covered with a nitrogen blanket;and the temperature was then increased to its target value at a rate of2° C. per minute, held at the target temperature for the desired lengthof time, and finally allowed to cool to room temperature by turning offthe heat in the oven. Some particles from each sample were sent toImpact Analytical for the measurement of T_(g) via DSC.

Particles of 14/16 U.S. mesh size were isolated from Sample 40m200C bysome additional sieving. This is a very narrow size distribution, withthe particle diameters ranging from 1.19 mm to 1.41 mm. This nearlymonodisperse assembly of particles was sent to FracTech Laboratories forthe measurement of the liquid conductivity of its packings.

D. Reference Sample

A Reference Sample was also prepared, to provide a baseline againstwhich the data obtained for the particles of the invention can becompared.

The formulation and the fabrication process conditions used in thepreparation of the Reference Sample differed from those used in thepreparation of the examples of the particles of the invention in two keyaspects. Firstly, carbon black was not used in the preparation of theReference Sample. Secondly, post-polymerization heat treatment was notperformed in the preparation of the Reference Sample. Consequently,while the examples of the particles of the invention consisted of aheat-treated and carbon black reinforced thermoset nanocomposite, theparticles of the Reference Sample consisted of an unfilled andas-polymerized thermoset polymer that has the same composition as thethermoset matrix of the particles of the invention.

Some particles from the Reference Sample were sent to Impact Analyticalfor the measurement of T_(g) via DSC. In addition, particles of 14/16U.S. mesh size were isolated from the Reference Sample by sieving andsent to FracTech Laboratories for the measurement of the liquidconductivity of their packings

E. Differential Scanning calorimetry

DSC experiments (ASTM E1356-03) were carried out by using a TAInstruments Q100 DSC with nitrogen flow of 50 mL/min through the samplecompartment. Roughly nine milligrams of each sample were weighed into analuminum sample pan, the lid was crimped onto the pan, and the samplewas then placed in the DSC instrument. The sample was then scanned from5° C. to 225° C. at a rate of 10° C. per minute. The instrumentcalibration was checked with NIST SRM 2232 indium. Data analysis wasperformed by using the TA Universal Analysis V4.1 software.

DSC data for the heat-treated samples are shown in FIG. 7. T_(g) wasdefined as the temperature at which the curve for the heat flow as afunction of the temperature went through its inflection point. Theresults are summarized in TABLE 3. It is seen that the extent of polymercuring in Sample 10m220C is comparable to that in Sample 40m200C, andthat the extent of polymer curing in both of these samples has advancedsignificantly further than that in Sample 10m200C whose T_(g) was onlyslightly higher than that of the Reference Sample.

TABLE 3. Glass transitions temperatures (T_(g)) of the threeheat-treated samples and of the Reference Sample, in ° C. In addition tobeing an “as-polymerized” (rather than a heat-treated) sample, theReference Sample also differs from the other three samples since it isan unfilled sample while the other three samples each contain 0.5% byweight carbon black.

ISOTHERMAL HEAT TREATMENT SAMPLE IN NITROGEN T_(g) (° C.) ReferenceSample None 117.17 10m 200 C. For 10 minutes at a 122.18 temperature of200° C. 10m 220 C. For 10 minutes at a 131.13 temperature of 220° C. 40m200 C. For 40 minutes at a 131.41 temperature of 200° C.F. Liquid Conductivity Measurement

A fracture conductivity cell allows a particle packing to be subjectedto desired combinations of compressive stress (simulating the closurestress on a fracture in a downhole environment) and elevated temperatureover extended durations, while the flow of a fluid through the packingis measured. The flow capacity can be determined from differentialpressure measurements. The experimental setup is illustrated in FIG. 8.

Ohio sandstone, which has roughly a compressive elastic modulus of 4Mpsi and a permeability of 0.1 mD, was used as a representative type ofoutcrop rock. Wafers of thickness 9.5 mm were machined to 0.05 mmprecision and one rock was placed in the cell. The sample was split toensure that a representative sample is achieved in terms of its particlesize distribution and then weighed. The particles were placed in thecell and leveled. The top rock was then inserted. Heated steel platenswere used to provide the correct temperature simulation for the test. Athermocouple inserted in the middle port of the cell wall recorded thetemperature of the pack. A servo-controlled loading ram provided theclosure stress. The conductivity of deoxygenated silica-saturated 2%potassium chloride (KCl) brine of pH 7 through the pack was measured.

The conductivity measurements were performed by using the followingprocedure:

-   1. A 70 mbar full range differential pressure transducer was    activated by closing the bypass valve and opening the low pressure    line valve.-   2. When the differential pressure appeared to be stable, a tared    volumetric cylinder was placed at the outlet and a stopwatch was    started.-   3. The output of the differential pressure transducer was fed to a    data logger 5-digit resolution multimeter which logs the output    every second during the measurement.-   4. Fluid was collected for 5 to 10 minutes, after which time the    flow rate was determined by weighing the collected effluent. The    mean value of the differential pressure was retrieved from the    multimeter together with the peak high and low values. If the    difference between the high and low values was greater than the 5%    of the mean, the data point was disregarded.-   5. The temperature was recorded from the inline thermocouple at the    start and at the end of the flow test period. If the temperature    variation was greater than 0.5° C., the test was disregarded. The    viscosity of the fluid was obtained from the measured temperature by    using viscosity tables. No pressure correction is made for brine at    100 psi. The density of brine at elevated temperature was obtained    from these tables.-   6. At least three permeability determinations were made at each    stage. The standard deviation of the determined permeabilities was    required to be less than 1% of the mean value for the test sequence    to be considered acceptable.-   7. At the end of the permeability testing, the widths of each of the    four corners of the cell were determined to 0.01 mm resolution by    using vernier calipers.    The test results are summarized in TABLE 4.    TABLE 4. Measurements on packings of 14/16 U.S. mesh size of Sample    40m200C and of the Reference Sample at a coverage of 0.02 lb/ft².    The conductivity (mDft) of deoxygenated silica-saturated 2%    potassium chloride (KCl) brine of pH 7 through each sample was    measured at a temperature of 190° F. (87.8° C.) under a compressive    stress of 4000 psi (27.579 MPa).

Reference Sample Sample 40m 200 C. Time (hours) Conductivity (mDft) Time(hours) Conductivity (mDft) 27 1179 45 1329 49 1040 85 1259 72 977 1091219 97 903 133 1199 120 820 157 1172 145 772 181 1151 168 736 205 1126192 728 233 1110 218 715 260 720

These results are shown in FIG. 9. They demonstrate clearly theadvantage of the particles of the invention in terms of the enhancedretention of liquid conductivity under a compressive stress of 4000 psiat a temperature of 190° F.

What is claimed:
 1. A method for forming a gravel pack within a wellborecomprising: (a) blending into the gravel pack formulation an effectiveamount of a polymeric nanocomposite particle, comprising: a polymermatrix; and from 0.001 to 60 volume percent of nanofiller particlespossessing a length that is less than 0.5 microns in at least oneprincipal axis direction; said nanofiller particles comprising at leastone of fine particulate material, fibrous material, discoidal material,or a combination of such materials, said nanofiller particles beingselected from the group consisting of natural nanoclays, syntheticnanoclays or mixtures thereof; wherein said nanofiller particles aresubstantially dispersed throughout said polymeric nanocompositeparticles, wherein said polymeric nanocomposite particle has a diameterranging from 0.1 mm to 4 mm; and (b) introducing the gravel packformulation with said effective amount of the polymer nanocompositeparticle into the wellbore.
 2. The method of claim 1, wherein saidnanofiller particles possess a length that is less than 0.5 microns inat least one principal axis direction and an amount from 0.1% to 15% ofsaid polymeric nanocomposite particle by volume.
 3. The method of claim1, wherein said polymeric nanocomposite particle is a spherical bead. 4.The method of claim 1, wherein said polymer matrix comprises at leastone of a thermoset epoxy, a thermoset epoxy vinyl ester, a thermosetpolyester, a thermoset phenolic, a thermoset polyurethane, a thermosetpolyurea, a thermoset polyimide, or mixtures thereof.
 5. The method ofclaim 1, wherein said polymer matrix comprises a terpolymer.
 6. Themethod of claim 5, wherein said polymer matrix is astyrene-ethylvinylbenzene-divinylbenzene terpolymer.
 7. The method ofclaim 1, wherein said nanofiller is natural nanoclays.
 8. The method ofclaim 1, wherein said nanofiller is synthetic nanoclays.
 9. The methodof claim 1, wherein said nanofiller is a mixture of natural andsynthetic nanoclays.
 10. The method of claim 1, wherein said polymericnanocomposite particle is blended with other solid particles includingat least one of sand, resin-coated sand, ceramic and resin-coatedceramic.